Such a combined reforming process is known from patent publication U.S. Pat. No. 6,100,303. In this document a process for making synthesis gas for subsequent use in methanol production is described, wherein a desulphurised feedstock gas consisting of a hydrocarbon gas having an H/C atomic ratio of 3-4, for example natural gas composed mainly of methane, is reformed using a combination of 3 different reforming reactors. The feedstock is first mixed with steam and then fed to a combustion-type (also called fired) steam methane reforming reactor (steam methane reformer, hereinafter abbreviated as SMR) and a heat-exchange type steam reforming reactor that is heated with hot gasses produced elsewhere in the process (also called gas heated reformer, hereinafter abbreviated as GHR), which two reactors are operated in parallel arrangement. The effluent gasses from SMR and GHR are mixed and fed to a secondary reforming unit together with oxygen, wherein the gasses undergo a catalytic partial oxidation reaction under essentially adiabatic conditions in addition to further reaction with steam. This reforming reactor is also referred to as auto-thermal reformer (abbreviated as ATR), as the excess heat generated by exothermic reaction is used to supply heat for the endothermic steam reforming reaction. The SMR unit is heated by burning part of the hydrocarbon feedstock gas and a purge of synthesis gas. The feed ratio of feedstock gas to the SMR and GHR units can vary from 1-3 to 3-1.
In the past decades, numerous processes have been developed to produce synthesis gas (also referred to in short as syngas) as one of the most important feedstocks in chemical industry. Syngas is a gaseous mixture containing hydrogen (H2) and carbon monoxide (CO), which may further contain other gas components like carbon dioxide (CO2), water (H2O), methane (CH4), and nitrogen (N2). Natural gas and (light) hydrocarbons are the predominant starting material for making synthesis gas. Syngas is successfully used as synthetic fuel and also in a number of chemical processes, such as synthesis of methanol or ammonia, Fischer-Tropsch type and other olefin syntheses, hydroformulation or carbonylation reactions (oxo processes), reduction of iron oxides in steel production, etc. The composition of synthesis gas, and thus its suitability for subsequent use for e.g. methanol production, is characterized mainly by its hydrogen and carbon monoxide content; generally presented by the so-called stoichiometric number (SN), which is defined asSN=([H2]−[CO2])/([CO]+[CO2])wherein the concentrations of components are expressed in vol % or mol %.
The value of SN is highly dependent on the reforming process technology used to make syngas. An overview of different technologies and their advantages and limitations is for example given by P. F. van den Oosterkamp in chapter “Synthesis Gas Generation: Industrial” of the “Encyclopedia of Catalysis” (John Wiley & Sons; posted on-line 2002/12/13, available via DOI: 10.1002/0471227617.eoc196).
The conventional technology for producing syngas from a methane feedstock is the reaction with water (steam) at high temperatures, generally called hydrocarbon steam reforming.
If a feedstock is used in a reforming process that is rich in higher hydrocarbons, like naphtha, the feedstock first needs to be treated in a so-called pre-reforming step, in order to convert the heavy hydrocarbons in the feed into methane, hydrogen and carbon oxides. Such higher hydrocarbons are more reactive than methane in steam reforming, and would—if present in the feed—lead to carbon formation and thus to deactivation of the catalyst employed in steam reforming. In such a pre-reformer several reactions take place simultaneously; the most important being hydrocarbon steam reforming (1), water gas shift (2), and methanation (3) reactions, which can be represented, respectively, as:CnHm+nH2O⇄nCO+(m/2+n)H2  (1)CO+H2O⇄CO2+H2  (2)CO+3H2⇄CH4+H2OCO2+4H2⇄CH4+2H2O  (3)
Such a pre-reformer is typically operated adiabatically at temperatures between 320 and 550° C., and is generally referred to as an adiabatic pre-reformer (hereinafter abbreviated as APR).
In a steam methane reformer (SMR) methane-rich gas is converted into a mixture containing carbon monoxide, carbon dioxide, hydrogen and unreacted methane and water in the so-called steam reforming (4) and carbon dioxide reforming (5) reactions, represented as:CH4+H2O⇄CO+3H2  (4)CH4+CO2⇄CO+2H2  (5)
These reforming reactions are strongly endothermic, while the accompanying water gas shift reaction is moderately exothermic. Such process thus calls for a reactor wherein heat management is extremely important. For the steam reforming process, several types of reactors are possible, such as the conventional widely used top-fired or side-fired reformers. In practice, a SMR unit may contain from 40 up to 1000 tubes, each typically 6-12 m long, 70-160 mm in diameter, and 10-20 mm in wall thickness. These tubes are vertically placed in a rectangular furnace or firebox, the so-called radiant section. The reactor tubes contain nickel-based catalyst, usually in the form of small cylinders or rings. The reactor tubes are fired by burners, which may be located at the bottom, at the side, or at the top of the furnace. Combustion of the fuel takes place in the radiant section of the furnace. After the flue gas has supplied its heat to all the reactor tubes, it passes into the convection section where it is further cooled by heating other streams such as process feed, combustion air, and boiler feed water as well as producing steam. The product gas, typically leaving the reformer at a temperature of 850-950° C., is cooled in a process gas waste heat boiler to produce process steam for the reformer. The syngas made with conventional steam reforming typically has a SN of between 2.6 and 2.9. For methanol production a composition having SN coming close to the theoretical value of 2 is preferred. The SN value of the syngas composition can be lowered by for example adding carbon dioxide; or by combined reforming (see below).
Steam reforming can also be performed in reactors wherein the necessary heat is supplied by heat exchange rather than by direct firing, for example by convective heat transfer from hot flue gasses and/or from hot syngas produced at another stage of a process. Several reactor concepts have been developed for this purpose, the name gas heated reformer (GHR) generally being used for a reactor that makes use of the heat present in syngas that is being produced in an auto-thermal reforming unit (ATR) or in a partial oxidation reformer; see below.
In an ATR catalytic conversion of a methane feedstock with oxygen (as pure oxygen, air, or enriched air) takes place in combination with the conversion with steam; an ATR is basically a combination of SMR and partial oxidation technology. In addition to the reactions mentioned above, also following strongly exothermic partial oxidation reactions (6) take place:CH4+½O2⇄CO+2H2 CH4+ 3/2O2⇄CO+2H2O  (6)
Desulphurised feedstock mixed with steam is introduced into the ATR reactor, as is oxygen in an appropriate amount. The upper part of the reactor basically consists of a burner mounted in the reactor shell. The exothermic reaction with oxygen delivers the endothermic heat of reaction of the steam reforming reaction, such that the overall reaction is auto-thermal, and takes place in the upper part; whereas the catalyzed reforming reaction takes place in a fixed bed in the lower part. Operating temperatures are relatively high, typically up to 1000° C., enabling very low amounts of unconverted methane in the product gas. The syngas produced in an ATR has a relatively low concentration of hydrogen; for subsequent methanol production mixing with hydrogen from another source will be needed.
Several process schemes that combine a steam reformer and an ATR unit (or a partial oxidation reactor) in different lay-outs have been proposed. Advantages of such combined reforming processes include controlling the SN of the syngas made at a targeted value.
As already indicated above, a combination of ATR and GHR technologies makes more efficiently use of energy; a further advantage being that adjustment of the SN of the syngas, e.g. close to the value of 2 as desired for methanol production, is possible.
WO 93/15999 A1 describes a process of making syngas by dividing a feed gas stream over a steam reformer and a partial oxidation reactor, and feeding the combined effluent streams to a second steam reformer.
In U.S. Pat. No. 4,999,133 a process for making syngas suitable for methanol production is disclosed, wherein a part of the feed is passed over a steam reformer, and resulting effluent and the other part of feed are fed to an ATR unit.
U.S. Pat. No. 5,512,599 relates to a process for making methanol from syngas on a large scale and with high energy efficiency, comprising a first step of steam reforming a hydrocarbon feed in a GHR reactor, followed by a partial oxidation and a second steam reforming step in an ATR unit, wherein effluent gas from ATR is used as a heat source for the GHR.
U.S. Pat. No. 6,444,712 B1 discloses a process for making methanol from syngas, wherein a methane feed is split and supplied to an ATR and a SMR operated in parallel, and effluent streams are fed to subsequent methanol synthesis. Unreacted syngas is recovered from methanol effluent and used for making hydrocarbons.
U.S. Pat. No. 5,496,859 discloses a process for making methanol from syngas, wherein a desulphurised methane-rich feed is supplied to an ATR and a SMR operated in parallel, and effluent streams are combined and fed to a second ATR to result in a syngas of proper composition and pressure for subsequent methanol synthesis.
EP 0522744 A2 describes a process for making a.o. methanol from syngas, wherein a desulphurised hydrocarbon feedstock is divided into 2 streams, of which a first stream is fed to a SMR unit, and a second stream is fed to an APR and a partial oxidation reactor operated in series, followed by cooling and mixing both reformed streams.
US 2004/0063797 A1 describes a process for making syngas especially suited for subsequent use in Fischer-Tropsch synthesis, wherein a desulphurised hydrocarbon feedstock is reformed in an APR, one or more steam reformers and an ATR, which are all operated in series.
EP 1403216 A1 describes a process for making syngas especially suited for subsequent use in Fischer-Tropsch synthesis, wherein a desulphurised hydrocarbon feedstock is reformed in one or more steam reformers and an ATR, which is operated in parallel with the other reformers.
GB 2407819 A discloses a process of making syngas from a hydrocarbon, e.g. natural gas, which process employs a combination of 3 reforming units, wherein the feed first passes an APR and then is split and fed to SMR and ATR units operated in parallel; to enable high syngas production capacity.
EP 1241130 A1 discloses a process of making syngas from light natural gas, by first treating the feed in an APR unit at 500-750° C. with a special catalyst, followed by conventional steam reforming; in order to reduce the heat supply required in steam reforming.
In EP 0440258 A2 a steam reforming process with improved reutilisation of heat is proposed, wherein a desulphurised hydrocarbon feed is first reacted in a first GHR, and the gas stream is then divided into 2 parallel streams, of which the first stream is fed to a SMR and the second stream to a further GHR, after which the effluent streams are combined and fed to an ATR unit.
EP 0959120 A1 discloses combined steam reforming processes aiming to optimise the energy efficiency by using heat from combustion gasses, including a scheme wherein GHR, SMR and ATR units are operated in series, and a scheme wherein the feed is fed to GHR and SMR units operated in parallel followed by reacting the combined effluent streams in an ATR.
WO 2005/070855 A1 describes an integrated process for making methanol and acetic acid from natural gas, wherein syngas is made by reacting part of the feed gas in an APR and SMR operated in series, and reforming the effluent combined with the remainder of the natural gas in an ATR.
In WO 2008/122399 A1 a combined reforming process for making a syngas mixture from a desulphurised methane-rich gaseous feedstock is disclosed, wherein the gaseous feedstock is mixed with steam and passed through an APR, and wherein the pre-reformed gas is then divided into three streams that are fed to a SMR, a GHR and to an ATR, which reforming reactors are operated in parallel. This process is stated to allow designing a methane-to-methanol production plant with a capacity of at least 10000 mtpd using available reforming equipment.
Methanol is one the most important chemical raw materials; in 2000 about 85% of the methanol produced would be used as a starting material or solvent for synthesis, whereas its use in the fuel and energy sector has been increasing rapidly. Since the 1960's, methanol synthesis from sulphur-free syngas with Cu-based catalysts has become the major route, as it can be operated at fairly mild reaction conditions. An overview of such methanol processes can be found in for example in the chapter “Methanol” in “Ullmann's Encyclopedia of Industrial Chemistry” (Wiley-VCH Verlag; posted on-line 2000 Jun. 15, available via DOI: 10.1002/14356007.a16—465).
Regarding the increasing demand for fuel and energy, there is a need in industry for ever larger and more efficient methanol production plants. Presently operated integrated production processes for making methanol from hydrocarbon feedstock typically have a maximum single line capacity on the order of 5000-7000 mtpd (metric ton per day). Practical limitations are encountered especially in syngas production, i.e. in the maximum size of available reforming reactors and of air separation units producing the required oxygen.
For example, limitations in the maximum size of a SMR unit lay in the number of tubes, in even gas distribution, and in heat transfer. About 1000 tubes is considered to be the maximum for a single unit operation, otherwise it will not be possible to control uniform distribution of gasses and thus of heat to all tubes. Reliability of all units is paramount, as minimizing down-time is a prerequisite for economical operation. Further capacity limitation results from a certain maximum amount of energy that can be transferred to the tubes. It is thus estimated that a technically and economically feasible SMR reactor of maximum capacity is characterized by a maximum reforming heat load of about 1150 GJ/h.
Production capacity of a GHR unit is mainly limited by a practical maximum in energy input by heat exchange with hot gasses; which is estimated to be about 420 GJ/h.
Presently available ATR or other partial oxidation units do not have the above limitations of steam reformers, but the maximum production capacity in this case is in practice limited by the volume of oxygen that is available. In most cases, oxygen is to be supplied from an air separation unit (abbreviated as ASU). The maximum size of a single state-of-the-art ASU is for technical and economical reasons considered to be about 4000 mtpd; which is equivalent to about 5200 kmol/h of oxygen. The equivalent maximum methanol production capacity based on such a single partial oxidation unit would be about 4500-6000 mtpd.
Although integrated production processes for making methanol from hydrocarbon in practice have a maximum single line capacity of about 6000 mtpd, schemes for larger scale plants have been proposed. Such schemes, however, typically employ operating units that exceed one of the above discussed maxima and practical limitations.